During fabrication of dynamic random access memory (DRAM) integrated circuit die on a semiconductor wafer, it is desirable to include a nonvolatile storage element which can be programmed during wafer probe and device testing. For example, programming a nonvolatile storage element could be used to identify the DRAM die, to reconfigure a tested DRAM array having defective memory cells into a smaller DRAM array having only functional cells, or even to remap defective DRAM memory cell addresses so that functional redundant cells are addressed instead.
There are several ways to implement nonvolatile storage on a DRAM integrated circuit die. For example, fusible links could be fabricated and data represented by using a laser to selectively create open circuits in the links. Such a nonvolatile memory is not reprogrammable since the vaporized fusible links cannot be reliably restored.
Laser trimming involves precise control of the power and position of the focused energy. It is more convenient to electrically program nonvolatile memory during wafer probe without using a laser. Fuses exist which can be selectively electrically programmed by exceeding a certain current and thereby creating an open circuit in the fuse. Antifuses can also be selectively electrically programmed by applying a voltage to break down a dielectric material contacted by two conductive terminals of the antifuse. This permanently changes the resistance presented by the antifuse from a high resistance to a low resistance.
Both fuses and antifuses implement non-reprogrammable nonvolatile memory. For example, if the wrong die identification data is programmed, this data is permanently associated with the programmed die.
Electrically erasable and programmable read only memory (EEPROM) techniques also implement nonvolatile memory on integrated circuits. EEPROMs can be electrically programmed, erased, and reprogrammed. One technique of implementing an EEPROM is by use of a floating gate tunneling oxide (FLOTOX) transistor. To create a FLOTOX transistor, a field-effect transistor (FET) having source, drain, substrate, and gate terminals is modified to electrically isolate (float) the gate. This polycrystalline silicon (polysilicon) floating gate is created over a thin insulating layer of silicon dioxide (gate oxide). A second polysilicon gate (control gate) is created above the floating gate. The floating gate and control gate are separated by an interpoly insulating layer.
Since the floating gate is electrically isolated, any charge stored on the floating gate is trapped. Storing sufficient charge on the floating gate will create an inversion channel between source and drain of the FET. Thus, the presence or absence of charge on the floating gate can represent two distinct data states.
FLOTOX transistors are selectively programmed by transferring electronic charges through the thin gate oxide onto the floating gate by Fowler-Nordheim tunneling. With the substrate voltage held at ground, the control gate is raised to a sufficiently high positive voltage so that electrons are transferred from the substrate to the floating gate by tunneling through the insulating thin gate oxide. The Fowler-Nordheim tunneling process is reversible. The floating gate can be erased by grounding the control gate and raising the drain voltage to a sufficiently high positive voltage to transfer electrons out of the floating gate to the drain terminal of the transistor by tunneling through the insulating gate oxide. The voltage applied to the control gate during programming is higher than the voltage applied to the drain during erasure because, while the erasure voltage is applied directly across the gate oxide, the programming voltage is applied to the control gate and capacitively coupled onto the floating gate.
The transistors can be selectively reprogrammed in the same manner as described above, since the Fowler-Nordheim tunneling process is nondestructive. The programming and erasure voltages which effect Fowler-Nordheim tunneling are higher than the voltages normally used in reading the memory. The Fowler-Nordheim tunneling effect is negligible at the lower voltages used in reading the memory, allowing a FLOTOX transistor to maintain its programmed state for years if subjected only to normal read cycles.
Since reprogrammable nonvolatile memory is useful for DRAM die identification and reconfiguring and remapping defective DRAM memory cells, it is desired to implement EEPROM through floating gate transistor structures which are compatible with existing DRAM processing steps.